Applicant's invention relates to a device for collecting optical energy, for example, portions of a laser beam, passing through an annular area and concentrating the collected energy to a point or a small-radius area. Compared to a conventional collector, such as a lens, that gathers energy passing through a circular area, the subject device reduces the effects of scintillations due to changes in the medium through which the optical energy passes by collecting energy from an area that is larger than the scale-size of such scintillations. Thus, the fluctuations in the various portions of the collected energy are uncorrelated and average out. The subject device also averages out speckle effects due to spatial incoherence of the laser beam.
Directed laser beams, when going through the atmosphere, are subject to a phenomenon called scintillation. Scintillation will cause a mottled pattern if the laser beam is viewed on a screen at some distance from the source, for example, a distance greater than about 2 kilometers. FIG. 1 illustrates the mottled pattern 5 in which different intensity levels are depicted by different cross-hatching. The pattern is in a constant state of flux, constantly "swimming" around at an audio frequency rate, due to air fluctuations. The sources of scintillation include winds, temperature gradients, aerosol makeup, humidity, and other attributes of an atmosphere that are absent in a vacuum.
For purposes of background, the following materials are noted: "A Survey of Clear-Air Propagation Effects Relevant to Optical Communications," Proc. IEEE, vol. 58, pp. 1523-45 (October 1970) by R. Lawrence et at.; Applications of Interferometry, 4th ed., pp. 19-22 (John Wiley and Sons, New York, 1954) by W. Williams; and Soviet Patent Application No. SU 1543370 (Feb. 15, 1990). Also, general techniques useful in optics are presented in many texts, for example, in Optics (Addison-Wesley, 1974) by E. Hecht and A. Zajac.
Lawrence et at. discloses that intensity fluctuations, or scintillations, in optical beams propagating through the atmosphere are well known and a subject of extensive study in astronomy and communications. Lawrence et at. also describes a technique of "smoothing", which involves combining uncorrelated intensity fluctuations to obtain an intensity that is a spatial average over the scintillations. Lawrence et al. notes that astronomical "seeing" is an "ill-defined" function of several factors, one of which is scintillation.
Williams describes the well-known Michelson stellar interferometer. Light from a star is collected by two widely separated mirrors and directed through a lens (or other concentrator) to a focal point where the beams from the two mirrors interfere. Williams discloses that the interference fringes can be distinct even when the star's image is "boiling".
The Soviet application describes a mirror-type optical concentrator in which an annular portion of the input light reflects from a primary mirror to a secondary mirror and then to a detector. Similar reflection paths are also seen in Gregorian and Cassegrainian telescopes.
These publications describe devices that collect energy from widely separated areas, but in general they seek to preserve the coherence of the collected energy for imaging or other purposes. It is not necessary to preserve such coherence, however, when all that is needed is a measure of a parameter such as average power and/or frequency. Accordingly, an energy concentrator/reflector that averages out scintillations and/or speckling effects and that can conform to a curved (non-flat) surface, such as an airfoil and the like, is highly desirable in the use of laser beams propagating through the atmosphere.